Japanese Laid-open Patent Publication No. 2005-102316 and Japanese Laid-open Patent Publication No. 2005-130453 disclose a configuration in which apparatuses on LAN (Local Area Network) at home are coupled to apparatuses on Internet via a router.
Addressing systems allocated to the networks differ between the apparatus on the LAN and the apparatuses on the Internet. The router performs an address conversion by using a NAT entry associating a private IP address of the LAN and a global IP address of the Internet, which enables communications (i.e., NAT traversal) between the apparatuses on the LAN and the apparatuses on the Internet.
Techniques related to the NAT traversal includes, for example, a port forwarding with a static NAT/UPnP (Universal Plug and Play) and a dynamic NAT/STUN (Simple Traversal of UDP through NATs). The port forwarding with the static NAT/UPnP sets a fixed NAT entry with respect to a NAT router, or sets a fixed port forward with UPnP with respect to an UPnP-capable router. The dynamic NAT dynamically generates the NAT entry at the communication via the Internet. In this case, the NAT entry is deleted upon no communication being performed for a certain time.
However, after the NAT entry has been deleted, the IP addresses cannot be associated in the router, which disables the communication between the apparatuses on the LAN and the apparatuses on the Internet.
On the other hand, the NAT entry set by the UPnP-capable router is not deleted. However, the UPnP technique cannot be used in the apparatus and its neighboring router on the LAN.